Hacking a ThinkPad Charging Cable for Use with a Portable Battery Pack

Victor Chew explains how he modified a USB-C to Slim Tip adapter cable to charge ThinkPad T450s.

Cameron Coward
14 days ago

If you’ve ever had the misfortune of a laptop charger breaking, then you know how frustrating they can be to replace. There is virtually no standardization among charger designs and even laptops from the same manufacturer and model line can require different chargers. That makes it difficult to find generic replacements, which forces you to purchase expensive official chargers — if they’re even still available. Victor Chew was in a similar situation, and explains how he hacked a Lenovo ThinkPad T450 charger cable for use with a portable battery pack.

A year ago, Chew had purchased an adapter cable for their wife’s ThinkPad X1 Carbon that would let her charge the laptop from a portable battery pack. That worked as intended, so he decided to purchase a similar adapter for their own ThinkPad T450 laptops. The cable has a female USB-C connector on one end and a proprietary Lenovo Slim Tip connector on the other end. It was supposed to take 65W of power from a battery pack and use that to charge the laptop, but it just didn’t work. The ThinkPad T450 would simply act as if it wasn’t receiving enough power.

The problem turned out to be that Slim Tip connectors contain a small resistor that tells the laptop how much current to draw, and this cable had the wrong resistor. A 120 ohm resistor tells it to draw 45W, a 280 ohm resistor tells it to draw 65W, and a 550 ohm resistor tells it to draw 90W. This cable had a 550 ohm resistor, so it was trying to draw more current than the 65W battery pack could provide. The solution was to crack open the connector housing, remove the 550 ohm resistor, and then put a new 280 ohm resistor in its place (though a 170 ohm resistor was used, as it was on hand). That surprisingly simple hack worked perfectly, so you can follow the same procedure if you find yourself in a similar situation.

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